Recently, in accordance with the spread of internet, there are increasing demands for an optical communication system as high bit rate communication infrastructure. In order to introduce the high bit rate communication system into general homes and make it more popular, a technique to realize a low cost of an optical circuit component included in the optical communication system is necessary.
An optical waveguide, that is, a principal composing element of the optical circuit component, can be generally fabricated by forming a desired groove pattern on a glass substrate by a lithography technique and a dry etching technique typified by semiconductor process. Since an expensive fabrication apparatus is necessary in this method, however, it is disadvantageously difficult to reduce the cost of the optical waveguide. Therefore, as described in Patent Document 1, attention is now being paid to a method for forming a desired optical waveguide or the like on a glass by pressing a mold having a desired concavo-convex structure against the surface of a softened material of glass. In this method, a desired optical waveguide can be mass produced when a mold is prepared, and hence, the optical circuit component can be inexpensively provided. However, since this method should be performed at a high temperature and a high pressure, the mold needs to have heat resistance, rigidity and durability. A material satisfying this necessity is a WC alloy including tungsten (W) and carbon (C), that is, hard metals, as principal components.
A method for forming a fine pattern on the surface of a WC alloy is a metal machining method using a diamond cutting tool disclosed in Patent Document 1, but the dimension of a concavo-convex pattern cut on a mold by this machining method is several microns or more and this machining method is also restricted in processing uniformity. As a method for realizing processing of a concavo-convex pattern not only in the dimension range realized by the metal machining method using a diamond cutting tool but also with a concavo-convex dimension of 1 μm or less, a microprocessing technique employing the lithography technique and the dry etching technique is effective. With this technique, not only a fine concavo-convex pattern can be formed but also processing variation is small and a mold can be fabricated at a lower cost than in the metal machining method using a diamond cutting tool.
As a dry etching technique for a WC alloy, Patent Document 2 discloses that the WC alloy can be dry etched by using CF4 or SF6.
Now, the conventional dry etching method will be described with reference to FIGS. 9(a) and 9(b). As shown in FIG. 9(a), a reaction chamber 101 in which a reduced pressure can be kept is provided with a gas inlet 102 and a gas outlet 103. Also, a plasma generator 104 for changing a gas supplied through the gas inlet 102 into plasma is provided in an upper portion of the reaction chamber 101. Furthermore, an electrode 106 on which a target material, specifically, a WC alloy substrate or a substrate having a WC alloy in its surface portion (hereinafter both referred to as a WC substrate 107), is placed is provided on an insulator 105 in a lower portion of the reaction chamber 101. An RF (radio frequency) power source 108 for applying a bias voltage to the electrode 106 is provided outside the reaction chamber 101.
Next, the operation of the etching system shown in FIG. 9(a) will be described by exemplifying the case where CF4 is used as an etching gas. As shown in FIG. 9(a), CF4 is introduced through the gas inlet 102 into the reaction chamber 101, and plasma 150 of the CF4 is generated by the plasma generator 104 and at the same time, RF bias is applied to the WC substrate 107 by the RF power source 108. As a result, radicals 109 of C, F or CFn (wherein n=1 though 4) and their ions 110 are produced in the plasma 150. At this point, in the plasma 150 used for the dry etching, the proportions in the number of atoms or molecules produced by the plasma 150 are generally in the order of F>CFn>>C. The radicals 109 are isotropically diffused to reach the WC substrate 107, but the ions 110 are accelerated between the plasma 150 and the WC substrate 107 and hence enter the WC substrate 107 substantially vertically. In particular, in the case where a F+ ion or a CFn+ ion including a F atom enters the WC substrate 107, a bond between W and C is cut and W is released in the form of WFx (wherein x=1 through 6). On the other hand, C is re-released in the form of CFy (wherein y=1 through 4).
The etching reaction caused on the surface of the WC substrate will now be described in more detail with reference to FIG. 9(b). As shown in FIG. 9(b), a resist pattern 112 is formed on a WC substrate 111. When the WC substrate 111 is etched with ions 113a and 113b of F+ or CF+ by using the resist pattern 112 as a mask, the W included in the WC substrate 111 is released in the form of WFx (wherein x=1 through 6) 114. At this point, the side face of a pattern of the WC substrate 111 obtained through the etching is in a bowing shape for the following reason:
In the etching of the WC substrate 111, most ions enter the WC substrate 111 substantially vertically like the ion 113a, but since ions basically have energy spread (an ion energy angular distribution), some ions enter the WC substrate 111 obliquely like the ion 113b. Accordingly, the anisotropic (vertical) etching of the WC substrate 111 by using the resist pattern 112 as the etching mask is realized by the ion 113a vertically entering the WC substrate 111. However, due to the impact caused by the ion 113b obliquely entering the WC substrate 111, the side face of the pattern of the WC substrate 111 is etched, resulting in the bowing shape as shown in FIG. 9(b).
Next, a conventional fine structure formation method for a WC alloy and a mold fabrication method by employing the same will be described with reference to FIGS. 10(a) through 10(d).
As shown in FIG. 10(a), a WC alloy substrate 121 is prepared, and a resist pattern 122 is formed on the WC alloy substrate 121 as shown in FIG. 10(b). The resist pattern 122 is generally formed by the lithography technique. Next, as shown in FIG. 10(c), a pattern is transferred onto the WC alloy substrate 121 by using the resist pattern 122 as a mask. At this point, the pattern transfer is performed by the dry etching technique.
When the aforementioned conventional dry etching technique is employed, since ions 123 entering the WC alloy substrate 121 from plasma have the energy spread, there are not only a component A vertically entering the surface of the WC alloy substrate 121 but also components obliquely entering the surface at an angle, namely, obliquely entering components B and C. Therefore, since the side face of a pattern of the WC alloy substrate 121 is etched by such obliquely entering ions, the etched cross-section is in what is called a bowing shape as shown in FIG. 10(c).
Then, the resist pattern 122 is removed through ashing, and the resultant substrate is cleaned. Thus, a mold made of the WC alloy substrate 121 having a fine concavo-convex structure in its surface and inside portions is obtained as shown in FIG. 10(d).
A conventional processing technique by using a mold is a nano-imprint method such as nano-imprint lithography proposed by S. Y. Chou et al. (see, for example, Patent Document 3 and Non-patent Document 1). In the nano-imprint method, a mold is pressed against a resist thin film formed on a semiconductor wafer for forming a fine resist pattern, and this method is currently under development for forming a fine pattern of a nano order as the minimum dimension. In a fine structure portion of a conventional mold for use in the nano-imprint method, a SiO2 film or a Si3N4 film that can be easily processed is used.
Patent Document 1: Japanese Patent No. 3152831
Patent Document 2: Japanese Laid-Open Patent Publication No. H1-98229
Patent Document 3: U.S. Pat. No. 5,772,905
Non-patent Document 1: Stephen Y. Chou, et al., Appl. Phys. Lett., Vol. 67, 1995, pp. 3114-3116